Nice challenge ... I would not use wood. It is organic and subject to weather. While it might seem odd, I would consider a metal-wood combination.

Check for a metal supplier and obtain an aluminum I-beam for your corners. You can attach the cable and turnbuckle to this and get the tension you want without worrying about future sagging. The look can be softened by bolting wood in the web spaces which can be stained or painted. An additional benefit is that the aluminum will not rot in the ground, preventing maintenance issues later.

1) a deep and sturdy base for the poles. 12' of height might need a 16' board.
2) 'knee-braced' against the sides of the patio as in your illustration
3) guy wires to counter the pull of the turnbuckles. (effective but potentially awkward)
4) if your yard is wooded, you could fly the lights from the trees

I don’t like to bring up the subject but what does your local building code have to say about a patio of that height and stairs without rails. The reason I bring it up I just built a deck around the same height and during construction I was enjoying it without the railings and like you I loved the openness of the space with the views. I also knew both code and our insurance carrier required more.

A couple weeks ago I toured the Frank Lloyd Wright House Falling Water and noticed the cantilevered decks that were 30 foot above the rocks below had rails barely to my knees and the public was allowed out on them with a warning first to watch your kids. I asked if this famous house could be built today with the codes and got an eye roll and most likely not was the answer. I assumed it was allowed because of what it is. But was really surprised that the center you come to before the house had an expanse of high decks that had just a single cable strung about 3 foot up. I’m not sure how they got away with that as little kids running around were under 3 foot high.

My deck by code had to have spindles spaced with 3.5” between them.

Getting back to your request. Check out a tractor supply store near you or a local feed mill or such. Ask about high tension fencing. It’s becoming really common for farmers and it has all the parts you will need.

Frankly, I wouldn't use a PT 4x4 because in a matter of a few days it will twist and bend. If anything I would use a 6x6x16'. Set in a good concrete base. I might even add 2- 6x6 legs, mounted at 45 degrees, attached about deck height and and buried in the ground, one running in front of the deck and one toward the house. You'll have a sort of tripod in a timber framed style. Paint to match the house.

@bud, Funny you mention that... we're in PA and the original owner of our home toured Falling Water and was inspired by it when he built this back in '46. I'm assuming the patio was built around then, so it gets grandfathered around modern code?

This was the original idea, BTW. While it definitely eases the tension between poles, it apparently increases the tension between spouses She likes her view of the backyard and doesn't quite see the need for so many. It's a compromise.